The present invention relates to a method of evaluating wear in engine oil, taking into account the addition of fresh oil.
To aid a driver, some software-based systems display information that it is time to change the engine oil in a motor vehicle, wherein the systems are based on algorithms which analyze parameters such as the distance traveled since the last oil change or the frequency of cold starts. Other methods rely on sensor signals which directly describe the physical condition of the oil, e.g., the dielectric constant of the oil or, as a much more reliable variable, the viscosity (xcex7) of the oil. A viscosity-based oil change criterion is derived from recording a change in viscosity since the last engine oil change, because wear on engine oil is usually associated with an increase in viscosity. For example, in one case, a viscosity value (xcex7max) may be defined as a limit value which must not exceed the viscosity value (xcex70) of the fresh oil as a reference value by more than a certain percentage, e.g., no more than 30%. The viscosity threshold above which engine oil should be changed and which is displayed for the driver of the vehicle is then given by the following equation, for example:
xcex7max=kxc3x97xcex70xe2x80x83xe2x80x83(1)
Factor k defines the allowed increase in viscosity, which corresponds to k=1.3 with a 30% allowed increase.
Adding fresh oil of a different type or a different viscosity class is problematical with these methods and similar methods of evaluating engine oil wear on the basis of viscosity measurements. Added oil has a different fresh oil viscosity, so the oil change criterion is subject to distortion. For example, the addition of engine oil of a higher viscosity class would simulate aging, although a de facto improvement in oil condition has been achieved by this addition of oil. Conversely, the addition of a lower-viscosity engine oil would simulate an improvement in the condition of the oil to an extent which does not conform to the actual condition. The fresh oil viscosity values for engine oil from adjacent viscosity classes typically differ by 30%.
The method according to the present invention allows evaluation of engine oil wear based on viscosity measurements, accounting for the addition of fresh oil, thereby permitting a correction of the reference value.
Specifically, the amount of oil added in topping off with fresh oil is detected by the oil level sensor, then the viscosity of the oil mixture (xcex72) of engine oil and fresh oil is determined by the oil viscosity sensor after an operating time and/or after reaching a predetermined oil temperature. The quantity (Vn) of the added fresh oil is determined by the electronic analyzer from a difference between the old oil level, i.e., oil volume (V1) and the new oil level, i.e., oil volume (V2). The viscosity (xcex7n) of the fresh oil is determined from the viscosity (xcex71) of the old engine oil immediately before the point in time of adding more oil and from the viscosity (xcex72) after the addition. A hypothetical initial viscosity (xcex7xe2x80x20) is calculated as a new corrected reference value from the known value of the viscosity (xcex70) of the old engine oil in the new condition, the quantity (V1) of the engine oil still present at the point in time of adding the new oil, the calculated viscosity (xcex7n) of the fresh oil and the calculated quantity (Vn) of the fresh oil. A new limit value (xcex7xe2x80x2max) for the viscosity threshold of the oil mixture is calculated from the corrected reference value, so that the oil should be changed after this limit value, which replaces the original value from (1), e.g., through the following computation rule:
xcex7xe2x80x2max=kxc3x97xcex7xe2x80x20xe2x80x83xe2x80x83(2)
To determine the viscosity (xcex7) of the added fresh oil and to determine the hypothetical initial viscosity (xcex7xe2x80x20) as a new corrected reference value, a mixing rule is used, where the value sought is a function of the known viscosities and the different oil quantities.
According to another exemplary embodiment of this method, additional parameters for the aging of oil may be accounted for as a viscosity threshold by the connected electronic analyzer in determination of the limit value of the maximum allowed viscosity (xcex7xe2x80x2max), e.g., the temperature gradient of heating of the oil, the number of cold starts or the length of trips traveled, and/or the operating times.
The determination of the corrected reference value (xcex7xe2x80x20) may be made more accurate by accounting for additional relationships and evaluation criteria and may even be stored as a reference characteristic in the electronic analyzer. For example, the temperature dependence of the viscosity may also be taken into account in the evaluation criteria.
To increase the accuracy of the reference value, additional evaluation methods may also be accounted for in evaluating engine oil wear, so that the engine oil quality of a vehicle may never fall below a required level without drawing the attention of the driver to an urgent need for an engine oil change.